How to Fit a New Cistern

What most people call a cold water tank is strictly a cold water cistern the difference is that a tank is a sealed vessel capable of containing water at above atmospheric pressure; a cistern is open to the atmosphere.

Types and sizes

There are three main types of cistern:

  • galvanised steel

These are heavy, relatively expensive, and prone to corrosion. Not a sensible choice for domestic work

  • rigid plastic

One the same size as an old galvanised cistern will make replacement work easier. Rigid plastic can withstand warm water and can be used as a feed-and-expansion cistern in central heating systems

  • flexible plastic

These are generally round and made of polythene. They can be squashed to get them through small loft hatches that rigid cisterns could not pass through. They should not be used as feed-and-expansion cisterns.

New Cistern

The normal recommended size for a cold water cistern is 50 gallons (227 litres) actual capacity cisterns that feed only a hot water storage cylinder or a central heating system could be smaller. In use, cisterns are never filled right to the brim so the nominal capacity (full to the brim) is more than the actual capacity   50 gal actual is 70 gal nominal.

To keep the water inside free from contamination, cisterns should have closely fitting, but not airtight lids. A suitable lid should exclude light and insects and should be unaffected by condensation. The cistern should be well insulated.

Fitting a new cistern

Great care should be taken while working in the loft

Turn off the water supply at the mains, drain the old cistern by turning on the taps in the bathroom and then bail out the water that remains in the bottom. You will be without water while you do the job so fill some kettles and sauce­pans first. Detach the old pipes – at the connections to the cistern if possible, par­ticularly if the new cistern is the same shape and size; otherwise, wherever con­venient. With old lead or rusty iron pipes, it may be necessary to saw through them. The old cistern will have to be left in the loft if it is too big to take out.

A new plastic cistern – flexible or rigid – must be well supported on a firm flat platform, positioned away from draughts and readily accessible.

Pipes are connected to the cistern using tank connectors a length of threaded pipe with a compression or capillary joint at one end. Pass the connector through a hole cut in the side of the cistern and hold it in place with nuts. A hole can be made with a hole saw or tank cutter or by making lots of little holes around the inside of the circumference of the hole you want with a drill, knocking out the ring, and then smoothing out the larger hole with a half-round file. Use plastic washers for sealing – jointing paste must not be used on plastic as it might crack it. It will be easier to make the holes and to fit the tank connectors before tak­ing the new cistern up into the loft. Once the ballvalve is fitted, run some water into the cistern to anchor it while the other fit­tings are made. Once the gate valves are fitted, the water can be turned on.

Pipes should be well supported so that the cistern is not taking any of the strain.

Outlets from the cistern should be about 50mm above the base of the cistern. The arm of the ballvalve should be adjusted so that the water surface is about 100mm from the top of the cistern.

Position the overflow pipe so that it dis­charges outside the house, in a con­spicuous position. It must be at least 22mm in diameter. The overflow pipe should be connected to the cistern about 25mm above the water level and about 20mm below the ball valve outlet.

The vent pipe from the hot water cylin­der should rise about 400mm above the cistern and terminate about 50mm above the water level. The hole in the lid should be fitted with a sleeve.

The final task is to insulate the cistern and all the pipes. Special kits are available for cistern insulation do not fit insula­tion under the cistern so that some heat can rise to prevent freezing.

Filed Under: Home & Maintenance


About the Author: Jason Prickett loves to write about home maintenance and stuff you can do yourself instead of hiring any professional. His step by step guides will assist you in completing your home maintenance tasks.

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